uclibc: context functions are not Thumb-compatible

uclibc: context functions are not Thumb-compatible

The context functions in uClibc unconditionally use the classic ARM
instruction set.

On platforms that do support the ARM instruction set, there is no problem -
However, on platforms that only support the Thumb instruction set, the
context functions cannot be built since the assembler code is not
Thumb-ready. Therefore, these functions must be disabled on such
platforms. All Thumb1 platforms support ARM instructions, so this is
only relevant for Thumb2-only platforms (i.e., Cortex-M).

Note that some packages require the context functions, so these will
fail to build on these platforms. It is worth mentioning that musl
also doesn't provide the context functions, and those are rarely
used. Affected packages will be handled in later patches.

[Peter: slightly reworded]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>