`man 3 signal`'s declaration has a face _only a mother could love_.

sighandler_t and __sighandler_t are not defined in the C standard, or POSIX.

They are helpful typedefs provided by glibc and the Linux kernel UAPI headers
respectively since working with function pointers' syntax can be painful. But
we should not rely on them; in C++ we have `auto*` and `using` statements.

Remove the proxy header, and only include a typedef for sighandler_t when
targeting Linux, for compatibility with glibc.

Fixes: #125598

These were created with operator new (see `Test::createCallable`), so operator
delete should be used instead of free().

Fixes: #122369
Fixes: #122378

Looks like the smart pointer I removed was being used to free the underlying
object.

Fixes: #122369

These make cross compiling the test suite more difficult, as you need
the
sysroot to contain these headers and libraries cross compiled for your
target.
It's straightforward to stick with the corresponding C headers.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This replaces the old macros LIBC_COPT_TEST_USE_FUCHSIA and
LIBC_COPT_TEST_USE_PIGWEED with LIBC_COPT_TEST_ZXTEST and
LIBC_COPT_TEST_GTEST, respectively.  These are really not about
whether the code is in the Fuchsia build or in the Pigweed build,
but just about what test framework is being used.  The gtest
framework can be used in many contexts, and the zxtest framework
is not always what's used in the Fuchsia build.

The test/UnitTest/Test.h wrapper header now provides the macro
LIBC_TEST_HAS_MATCHERS() for use in `#if` conditionals on use of
gmock-style matchers, replacing `#if` conditionals that test the
framework selection macros directly.

Fixes #88187

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079