Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Revert r353140: Re-add ALLOW_MIPS_SHARED_TEXTREL, sprinkle it around

arichardson has an actual fix for the same issue that this was working
around; given that we don't build with llvm today, go ahead and revert the
workaround in advance.

Re-add ALLOW_MIPS_SHARED_TEXTREL, sprinkle it around

Diff partially stolen from CheriBSD; these bits need -Wl,-z,notext in order
to build in an LLVM world. They are needed for all flavors/sizes of MIPS.
This will eventually get fixed in LLVM, but it's unclear when.

Reported by: arichardson, emaste
Differential Revision: https://reviews.freebsd.org/D21696

Run the csu tests on a DSO. This builds the tests into a shared library,
then runs these from the base test programs. With this we can check
crtbeginS.o and crtendS.o are working as expected.

MFC with: r339738
Sponsored by: DARPA, AFRL