stm32: f373: support the 373xC devices

These devices have 32kiB RAM and 256kiB flash and are otherwise
register compatible with the f373x8 and f373xB devices.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>
irq2nvic_h: don't declare irq symbols as WEAK

At least in some toolchains (e.g., Debian Jessie with gcc 4.8.3-9+11
and ld 2.24.51.20140604), declaring an irq symbol as WEAK together with
gcc -ffunction-sections / ld --gc-sections leads to the default version of
the IRQ function to be taken, rather than the user-supplied version.