Enable -mcmodel=medany by default on RiscV64

[jklmnn]

The RiscV64 runtimes (especially polarfiresoc) expect the compiler to use -mcmodel=medany. Otherwise they will produce relocation errors. Adding the switch manually fixes the problem for light-polarfiresoc and light-tasking-polarfiresoc. However the problem seems to persist for embedded-polarfiresoc. All three runtimes work with our compiler configured with --with-specs='%{!mcmodel=*:-mcmodel=medany}'.

Note that this problem does not seem to appear with the spike runtime as it still manually adds -mcmodel=medany for historical reasons.

This issue was initially reported in AdaCore/bb-runtimes#73.

cc @Fabien-Chouteau

[robdaemon]

As a note, I'm seeing this same behavior with the light-rv64imafdc runtime right now:

/home/rroland/.local/share/alire/toolchains/gnat_riscv64_elf_14.2.1_4f738310/riscv64-elf/lib/gnat/light-rv64imafdc/adalib/libgnat.a(s-textio.o): in function `system__text_io__get':
s-textio.adb:(.text.system__text_io__get+0x0): relocation truncated to fit: R_RISCV_HI20 against `.LANCHOR0'
/home/rroland/.local/share/alire/toolchains/gnat_riscv64_elf_14.2.1_4f738310/riscv64-elf/lib/gnat/light-rv64imafdc/adalib/libgnat.a(a-elchha.o): in function `__gnat_last_chance_handler':
a-elchha.adb:(.text.__gnat_last_chance_handler+0x2): relocation truncated to fit: R_RISCV_HI20 against `.LC4'

[AldanTanneo]

Flag added in #88 😄