read_csr() returns an unsigned long, not a uint32_t. This causes a
-Wformat warning to be emitted when compiling with clang. This commit
fixes the warning by changing the format string.
This requires -nostartfiles to be only passed to the linker, not the
compiler, as it is a linker flag and passing it to the compiler causes a
clang warning to be emitted.
Additionally, clang does not seem to support `-mcmodel=medlow` and
`-msmall-data-limit=8` but these options do not seem strictly necessary
to me anyhow thus they are deactivated conditionally when using clang.
