style: change C/C++ source permissions to noexec

Several files in the repository were tracked with execute permissions
even though the files are just normal C/C++ files (and the one .isa).

Change-Id: I976b096acab4a1fc74c5699ef1f9b222c1e635c2
Reviewed-on: https://gem5-review.googlesource.com/7241
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

MIPS: Get MIPS_FS to compile, more style fixes.
Some breakage was from my BitUnion change, some was much older.

MIPS: Use BitUnions instead of bits() functions and constants.
Also fix style issues in regions around these changes.

Add in files from merge-bare-iron, get them compiling in FS and SE mode

mips import pt. 1

src/arch/mips/SConscript:
"mips import pt.1".