[FPU] prevent GCC from emitting fused multiply-add instructions

[stnolting]

The FPU of the Zfinx ISA extension does not support any fused /multiply-add instructions:

• fmadd.s
• fmsub.s
• fnmsub.s
• fnmadd.s

In order to prevent GCC from emitting such instruction a new compiler flag is added (-ffp-contract=off).

Without -ffp-contract=off (or -ffp-contract=on, which is the default): ❌

;fx = (f0 * f1) + f2;
218:  00012783  lw       a5,0(sp)
21c:  00412683  lw       a3,4(sp)
220:  00812703  lw       a4,8(sp)
224:  70d7f7c3  fmadd.s  a5,a5,a3,a4
228:  00f12623  sw       a5,12(sp)

With -ffp-contract=off: ✔️

;fx = (f0 * f1) + f2;
218:  00012783  lw      a5,0(sp)
21c:  00412683  lw      a3,4(sp)
220:  00812703  lw      a4,8(sp)
224:  10d7f7d3  fmul.s  a5,a5,a3
228:  00e7f7d3  fadd.s  a5,a5,a4
22c:  00f12623  sw      a5,12(sp)