Since someone on Discord asked why macOS arm64 was not listed.

https://llvm.org/docs/GettingStarted.html#hardware

Add a few known platforms:
* Linux AArch64
* FreeBSD AArch64
* macOS arm64 (Clang build only, there might be a GCC port but I've not
used it myself)
* Windows on Arm (ARM64 as Microsoft refers to it)

This is a small change, but unblocks using longer string literals in
LLVM's source code, and hopefully isn't disruptive.

Discussed in an RFC here:
https://discourse.llvm.org/t/rfc-raising-minimum-msvc-version-by-one-dot-release/

...and add shared libs as a suggestion.

* Mark options, option values and program names as plain text.
* Add a blank line between the option and the explanatory text
  so that it doesn't get printed on the same line.
  (this seems to be the original intent of the rst source anyway)
* Update the phrasing of a couple of the options.
* Add BUILD_SHARED_LIBS to suggestions.

New Headergen requires PyYAML version 5.1 or newer in order to generate
header files .

This reverts commit b6824c9d459da059e247a60c1ebd1aeb580dacc2.

This relands commit 0a6c74e21cc6750c843310ab35b47763cddaaf32.

The original commit was reverted due to buildbot failures. These bots
should be updated now, so hopefully this will stick.

This reverts commit f2931182fc877e813974a5f53721f859bfb5b130.

This was again causing buildbot failures. #83962 has been updated with the new
failures, notifying the buildbot maintainers that they need to update their
bots.

This reverts commit 2dfa30d0ca6fb6991640a18e53401d82f567f8ff.

This relands commit 0a6c74e21cc6750c843310ab35b47763cddaaf32.

This patch originally caused a host of buildbot failures due to several
buildbots not having the necessary python version. That was tracked in
issue #83962, and all bots that failed in the first round have now been
updated. This is an attempt to reland the patch to see if it sticks or
if there are a number of long-running bots where this patch will cause
failures.

This allows to keep fetching release branches as well.

Add the ability to set the number of tablegen jobs that can run in
parallel
similar to the LLVM_PARALLEL_[COMPILE|LINK]_JOBS options that already
exist.

Following the changes made for:
- https://reviews.llvm.org/D154130: [lit][clang] Avoid realpath on Windows due to MAX_PATH limitations
in commit:
- 05d613ea931b6de1b46dfe04b8e55285359047f4

Python 3.8.0 or newer is now required by at least the following tests
when they are run on Windows from a substitute (virtual) drive. A
substitute drive is often used as a workaround for `MAX_PATH`
limitations on Windows. These tests are impacted because they use the
lit `%{?:real}` path expansion syntax to expand symbolic links and
substitute drives. This path expansion is implemented with Python's
`os.path.realpath()` function which changed behavior in Python 3.8.0
with regard to expansion of substitute drives. The changes mentioned
above rely on the newer Python behavior.
- `clang/test/Lexer/case-insensitive-include-absolute.c`
- `clang/test/Lexer/case-insensitive-include-win.c`

This change updates the LLVM Getting Started guide to note this newer
Python version dependency for this relatively niche case. Python 3.6.0
remains the minimum required Python version otherwise.