Avoid optimizing workload away

[7FM]

Clang without volatile:

8000111c <workload_task>:
8000111c: ff010113         addi    sp, sp, -0x10
80001120: 00112623         sw    ra, 0xc(sp)
80001124: 060000ef         jal    0x80001184 <no_task_yield>
80001128: ffdff06f         j    0x80001124 <workload_task+0x8>

With volatile:

8000111c <workload_task>:
8000111c: e5010113         addi    sp, sp, -0x1b0
80001120: 1a112623         sw    ra, 0x1ac(sp)
80001124: 1a812423         sw    s0, 0x1a8(sp)
80001128: 1a912223         sw    s1, 0x1a4(sp)
8000112c: 1b212023         sw    s2, 0x1a0(sp)
80001130: 19312e23         sw    s3, 0x19c(sp)
80001134: 19412c23         sw    s4, 0x198(sp)
80001138: 00000413         li    s0, 0x0
8000113c: 00810493         addi    s1, sp, 0x8
80001140: 51eb8537         lui    a0, 0x51eb8
80001144: e7000913         li    s2, -0x190
80001148: 51f50993         addi    s3, a0, 0x51f
8000114c: 00030a37         lui    s4, 0x30
80001150: 03343533         mulhu    a0, s0, s3
80001154: 00555513         srli    a0, a0, 0x5
80001158: 03250533         mul    a0, a0, s2
8000115c: 00a48533         add    a0, s1, a0
80001160: 01452023         sw    s4, 0x0(a0)
80001164: 068000ef         jal    0x800011cc <no_task_yield>
80001168: 00140413         addi    s0, s0, 0x1
8000116c: 00448493         addi    s1, s1, 0x4
80001170: fe1ff06f         j    0x80001150 <workload_task+0x34>