gmp: don't use uname to detect system features #51515
Motivation for this change
On 32-bit ARM, gmp is explicitly configured with an alternative way to autodetect the build platform to work around a bug in the default system detection. This has the side effect of making it impossible to build correctly for armv6l on an armv7l machine.
This PR replaces the
I split the change into two commits, the first which avoids a mass rebuild on architectures other than 32-bit ARM, and second which cleans up the derivation with a mass rebuild (removing the
I would like to get the first commit into master, and the second into staging, but I'm not sure what the procedure is for that.