Cleanup build makefile

[ZERICO2005]

FYI: common.mk is for building the toolchain, and makefile.mk is for users.

• Important Change: Made it so include/c++/math.h gets included before include/math.h as required by the C++ standard.
• cleaned up common.mk and makefile.mk.
• split the arguments across multiple lines to make merge/rebase conflicts easier to deal with.
• splitting the arguments also makes git blame/diff easier to read.
• Added -Wshadow and -Wimplicit-int-conversion to common.mk.
• -fno-threadsafe-statics was added to common.mk (prevents __cxa_guard_* functions from being emitted)

Cleaning up these makefiles also can help catch when they go out of sync

Other changes:

• fixed -Wshadow and -Wimplicit-int-conversion warnings
• fixed NULL dereference in fgetc/fputc when stream is NULL (caught with --analyze + some extra flags)