protect access to the gnu warning map with a mutex to avoid random crashes

discussed with, tested by and ok tb@

Stop erroring out when .gcc_except_table relocs point at discarded sections

lang/gcc on riscv64 has a wart, it creates such relocs which upset ld.lld.
The workaround I have used in lang/gcc since Sep 2022 doesn't work any
more, so ask ld.lld to be more lenient. This will let the fortran ports
and friends build on riscv64. clang has fixed since some time already,
but our lang/gcc port will likely keep that wart for some time.

Upstream report: https://reviews.llvm.org/D83244

Input and ok kettenis@

merge lld-16.0.6

add .gnu.warning.SYMBOL support to ld.lld(1) to display the warnings in
these sections like ld.bfd(1)

e.g:
add.c(add.o:(add)): warning: sprintf() is often misused, please use snprintf()
add.c(add.o:(add)): warning: strcpy() is almost always misused, please use strlcpy()
add.c(add.o:(add)): warning: strcat() is almost always misused, please use strlcat()

ok deraadt@

Merge LLVM 13.0.0.

Merge LLVM 11.1.0.

Backport https://reviews.llvm.org/D75419. This adds a workaround for
undefined references to .got2 on powerpc making it possible to link
llvm with lld.

ok gkoehler@

branches: 1.1.1;
Initial revision