Bootloader: Add Dockerfile to cross compile Linux bootloaders for abnormal architectures.

[bwoodsend]

Add a Dockerfile which leverages the 'dockcross' family of cross compiler base images to build the Linux bootloaders for almost any architecture.

This required a few preliminary changes:

• The waf script now always assumes it's building for the C compiler's reported target architecture if the target OS is Linux. Previously, it was determined from the host machine which of course is wrong in a cross compile setup.

• To avoid bootloaders of different architectures clobbering each other if present in the same installation or git clone, append an architecture family name in the path to the bootloader. e.g. A ppc64le (Power PC, 64 bit, little endian) bootloader would be saved under PyInstaller/bootloader/Linux-64bit-ppc/run. This change was already half implemented as a special case for ARM and sw_64. This change only applies to Linux. All other OSs either don't need this change or, in the case of macOS, have their own special requirements and handling. This use of a loose family name for architectures rather than the exact output from platform.machine() is needed so that waf can predict this name based only on the target reported by the C compiler which is considerably less exact than that of uname -m or platform.machine(). For example, when platform.machine() is armv5l, armv6l or armv7h, the C compiler simply reports arm in all cases. The mapping should go from armv7h to arm rather than the other way around.

A couple of unintended bonuses of this approach:

• dockcross provide manylinux2014 compatible compilers (see PEPs 513 and 599) to avoid the glibc symbol runtime errors that crop up if you don't keep glibc requirements lower than your target's version. This commit can also be used to build native x86_64 binaries using a non-cross variant which conform to the manylinux2014 standard (which, as of setup: Build per-platform wheels. #5787, is advertised by our wheel tags).

• dockcross also includes cross compilers for alternative libc implementations meaning that we could use this to build musl bootloaders for Alpine or Bionic bootloaders for Android if we wanted to. However, they'd be difficult to distribute because PyPI currently only allows glibc Linux binaries.

[bwoodsend]

I did some PyPI stats queries on Google Big Query to see is it's worth shipping wheels for any of these architectures. These are the download stats for the last month. The architectures with an asterisk next to them are the ones supported by manylinux and therefore PyPI.

Architecture	Count
x86_64 *	240961
aarch64 *	4095
armv7l	1979
i686 *	319
armv8l	133
ppc64le *	72
mips64	71
s390x *	62
armv6l	30
sw_64	7
ppc64	3
loongarch64	1

So I suggest that we start shipping wheels for aarch64 and maybe i686 for now. Although I'd rather we don't check the bootloaders into the repo like we do with the standard ones.