This is required for ISA 2.0 support, see chapter
"Zicsr", Control and Status Register (CSR) Instructions, Version 2.0
in
RISC-V Instruction Set Manual, Volume I: RISC-V User-Level ISA
LLVM/LLD does not support STARTUP and ALIGN_WITH_INPUT directives that
GNU LD support. INPUT and ALIGN(8) are supported by LLVM/LLD and can
replace the unsupported STARTUP/ALIGN_WITH_INPUT directives.
The commit conditionally adds the supported directive that linkers
can understand depending on the toolchain used to compile RTEMS
i.e., clang or gcc. Clang is assumed to use LLD by default.
