Fix Thumb asm directives to allow unwinding through SyscallAsm.
Fix Thumb asm directives to allow unwinding through SyscallAsm.

The BaselinePolicy.SIGSYS_InvalidSyscall fails with Thumb and
-fno-omit-frame-pointer due to a missing .save directive that
indicates where to find r7.

I also removed the usage of r7 as the CFA register since it seems
incorrect (since r7 is clobbered).

TEST=sandbox_linux_unittests --gtest_filter=BaselinePolicy.SIGSYS_InvalidSyscall --single-process-tests (on Chromecast)

Review URL: https://codereview.chromium.org/1121083002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#328155}

Just-D/chromium-1,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Just-D/chromium-1,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Just-D/chromium-1,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk