Makefile: Avoid repeated evaluation of shell commands

Recursively expanded variables (defined with '=') are expanded at
evaluation time. These version information variables are evaluated
inside a recipe as part of GENFLAGS. As a result, the shell commands
are executed separately for each compiler invocation. Convert the
version information variables to be simply expanded, so the shell
commands are executed only once, at Makefile evaluation time. This
speeds up the build by as much as 75%.

A separate check is needed to maintain the behavior of preferring the
value of OPENSBI_BUILD_TIME_STAMP from the environment.

Signed-off-by: Samuel Holland <[email protected]>
Reviewed-by: Xiang W <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Anup Patel <[email protected]>

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