[gn][fvm] Fix some warnings in host build

ninja: Entering directory `build-gcc'
[4399->238/4694 ~58] CXX host-x64-linux-clang/obj/system/host/fvm/fvm.main.cpp.o
../system/host/fvm/main.cpp:50:28: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
    for (unsigned i = 0; i < argc; i += 2) {
                         ~ ^ ~~~~
../system/host/fvm/main.cpp:128:14: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
    while (i < argc) {
           ~ ^ ~~~~
../system/host/fvm/main.cpp:129:50: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
        if (!strcmp(argv[i], "--slice") && i + 1 < argc) {
                                           ~~~~~ ^ ~~~~
../system/host/fvm/main.cpp:140:58: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
        } else if (!strcmp(argv[i], "--offset") && i + 1 < argc) {
                                                   ~~~~~ ^ ~~~~
../system/host/fvm/main.cpp:145:58: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
        } else if (!strcmp(argv[i], "--length") && i + 1 < argc) {
                                                   ~~~~~ ^ ~~~~
5 warnings generated.

BLD-361 #comment [gn][fvm] Fix some warnings in host build
ZX-3415 #comment [gn][fvm] Fix some warnings in host build

Test: CQ
Change-Id: Ib9b92867d1abb025af77b4ce47638dbaf3519d17