[libc][setjmp] implement sigsetjmp/siglongjmp

[nickdesaulniers]

When cross compiling the libc testsuite, I ran into:

llvm-project/libc/test/UnitTest/FPExceptMatcher.cpp:31:21: error: unknown type name 'sigjmp_buf'
   31 | static thread_local sigjmp_buf jumpBuffer;
      |                     ^

and @lntue points out the usage in https://github.com/llvm/llvm-project/blob/main/libc/test/UnitTest/FPExceptMatcher.cpp#L45 of the relevant functions.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

When cross compiling the libc testsuite, I ran into:

llvm-project/libc/test/UnitTest/FPExceptMatcher.cpp:31:21: error: unknown type name 'sigjmp_buf'
   31 | static thread_local sigjmp_buf jumpBuffer;
      |                     ^

and @lntue points out the usage in https://github.com/llvm/llvm-project/blob/main/libc/test/UnitTest/FPExceptMatcher.cpp#L45 of the relevant functions.