Add GCC compiler option. (#1056)

* GCC support.

* Add note about gcc-mips-linux-gnu requirement to README.md.

* changes

* changes based on Tharo's suggestion

* Cant reproduce file_choose.h error. Removing unnecessary -I

* changes

* wording based on Fig's suggestion

* add AVOID_UB for eyes/mouth reordering issue.

* remove unneeded flags and deprecate ZAPDFLAGS.

* some changes, waiting on prs for the rest

* fixes

* discard header sections

* change section handling in mkldscript

* avoid_ub in DmaMgr_GetFileNameImpl

* move asm to inline asm (consolidate gcc functions)

* change prefix back

* remove space

* fix warnings

* Revert "remove space"

This reverts commit 94af6977b3.

* Revert "fix warnings"

This reverts commit d729ddf457.

* finish up missing_gcc_functions

* revert unwanted mkldscript change

* temporary workaround. TODO: Stop the asm processor from choking

* fix ido build

* Revert "temporary workaround. TODO: Stop the asm processor from choking"

This reverts commit 9df892b7ac.

* review

* remove unused line in mkldscript

* remove tabs

* clarify zf comment

* review2

* review

* remove duplicate cc_check

* vanilla code always come first

* std_dma avoid ub

* add compiler_gcc to cflags

* only use two blocks when necessary

* clarify zf comment

Co-authored-by: fig02 <fig02srl@gmail.com>

src/boot/z_std_dma.c

@@ -210,6 +210,9 @@ const char* DmaMgr_GetFileNameImpl(u32 vrom) {
     }
     //! @bug Since there is no return, in case the file isn't found, the return value will be a pointer to the end
     // of gDmaDataTable
+#ifdef AVOID_UB
+    return "";
+#endif
 }
 
 const char* DmaMgr_GetFileName(u32 vrom) {