Add sh4 support to Thunderbird

Add sh4 support to Thunderbird

Cherry-picked and adapted patches from Firefox upstream:
- 5a74d13630a969473f72a143f37728f5bb87f552
Bug 1329194 - mozbuild: Add SH4 as target architecture.
- b179235c38eb631286f9e2fd4a7b23361dbab6d2
Bug 1329194 - js:jit: Use PowerPC atomic operations on SH.
- ba58645c87ed7e08b16adc3b40f0c6e3b6a2e140
Bug 1329194 - mfbt:tests: Define RETURN_INSTR for SH in TestPoisonArea.
- 942df3d3a4a1db3d182e97c54fb1705c50128470
Bug 1329194 - ipc:chromium: Add platform defines for SH.
- 85ae466ed90b3a0cac396aed24722ce4d0febb31
Bug 1329194 - media:webrtc: Add platform defines for SH.
- 5a74d13630a969473f72a143f37728f5bb87f552
Bug 1329194 - mozbuild: Add SH4 as target architecture.
- (not committed upstream yet)
Bug 382214 - xpcom: Add xptcall support for Linux/SH
- (not committed upstream yet)
Bug 382214 - xpcom: Make SH xpctcall inline assembly position-independent

Closes: #859508
Gbp-Pq: Topic porting-sh4
Gbp-Pq: Name Add-sh4-support-to-Thunderbird.patch